The invention relates to the devices which make it possible to display a television picture on a large screen and applies to television monitors and receivers.
Many devices of this type have been produced, but the results obtained hereto are not entirely satisfactory. There are, in particular, known projectors which comprise three cathode-ray tubes and an optical device projecting and superimposing on a diffusing screen three images supplied by these three cathode-ray tubes. This type of device is relatively economical, but has a low level of brightness, and it gives a picture of inferior quality as compared to that of a cathode-ray tube with a mask, conventionally used for pictures having a diagonal less than 80 cm. Its luminous efficiency is poor because of the small aperture of the optical projection device, and the contrast obtained is low as a result of the scattering caused by the optical device and by the cathode-ray screens themselves, because they have to supply a very high candle power. There are also known projection devices making use of light valves which give good results, but are difficult to use and are relatively expensive. There are also known projection devices utilizing laser beams, but their luminous efficiency is extremely low.
Another solution is to construct a display device with direct vision, but with very large dimensions. Large-size cathode-ray tubes have been constructed, but enormous technological difficulties arise in producing them on an industrial scale, when the screen diagonal exceeds 80 cm, and on the other hand the depth of the tube results in a considerable bulk. To avoid these disadvantages, it is known to use electron beams guided in rows and columns by electrodes of suitable shapes, but this solution appears difficult to put into practice on an industrial scale at the present time.
On the other hand, devices utilizing plasmas as electron sources to excite phosphors are now being studied. This process presents numerous problems: each element of the device must be controlled by a constant voltage which is pulse-length modulated; the constrast obtained is low (20:1); the candle power is approximately 5 times less than that of a conventional cathode-ray tube.
U.S. Pat. No. 4,368,485 describes a television-picture display device consisting of a plurality of small cathode-ray tubes of square shape which are placed next to one another to form a matrix. Each tube reproduces only 4 picture elements, thus making it possible to make the spaces attributed to the joints between the tubes unnoticeable, but the number of tubes, frame-deflecting coils and multiplexing circuits is extremely high. The complexity of this device results in a very high cost.
U.S. Pat. No. 3,909,525 describes a display device comprising a plurality of conventional trichromic cathode-ray tubes placed next to one another to form a matrix, each cathode-ray tube reproducing a rectangular position of the picture to be displayed and being associated with an optical device which enlarges this picture portion in such a ratio that the enlarged picture portions appear contiguous in spite of the space occupied by the joints between the tubes. This document does not describe means for supplying each tube with the video signals and scan signals necessary to display on each of them a specific portion of the picture to be displayed.
A device which is simple to produce can consist of conventional monitors, buffer memories and control means for controling reading and writing in these memories and synchronizing the scans of the monitors. A conventional monitor operating according to European standards executes a scan of 312.5 lines per field in 20 ms. For example, a complete picture comprising 625 lines can be reproduced on a matrix of 10.times.10 monitors, each monitor reproducing 62 picture lines, each picture line being reproduced by 10 scan lines of the monitor, the corresponding information being read 10 times from a buffer memory associated with the considered monitor.
The information corresponding to the bottom of a field is available only 20 ms after the start of analysis of this field. On the other hand, each picture portion requires 20 ms to be displayed, since the monitors are assumed to have a standard scan. In particular, the last picture portion located in the lower right-hand corner is displayed completely after a period of 20 ms in relation to the start of display of this picture portion, this start being delayed 9.times.62 lines+9 lines in relation to the moment when the picture begins to be displayed in the upper left-hand corner of the screen. The sum of these two delays is practically equal to 40 ms. The replacement of one picture by another therefore lasts 40 ms instead of the usual 20 ms.
When a sequence of fields represents objects moving at high speed, this period in which the displayed picture is renewed is very disturbing, since the eye perceives a delay between the changes in the top of the picture and those in the bottom of the picture. This simple device is therefore unsuitable for pictures with motion images. On the other hand, the brightness of the cathode-ray tubes is often insufficient for use in the open air, the more because it is reduced as a result of the optical device. This brightness is limited by a saturation of phosphors of the screen of the tubes, this saturation depending on the energy of the electron beam and its scanning speed.
The device according to the invention avoids these disadvantages, while at the same time making use of conventional techniques.